How do I write protect an external USB Drive so the files can't be erased.

I have an external USB drive that is formatted for Max OX Extended (Journaled). It has a backup of my music collection so I want to write protect it so that it can only be READ and not erased or any of the files erased. How can this be accomplished - running an iMac and El Capitan.
I have tried right clicking on the drive and the sub folders in Finder and changing the Read/Write attributes to READ ONLY for myself and everyone, but I can still delete individual files.
Thanks.

Well, I have checked it myself and have to agree with you, even you change the permissions one can still delete files. There's one thing which is helpful to prevent deleting files. And that is the setting "protected".